Much of the electrical power used in homes and businesses throughout the world is produced in power plants that burn a fossil fuel (i.e. coal, oil, or gas) in a boiler. The resulting hot exhaust gas (also sometimes termed “flue gas”) turns a gas turbine or boils water to produce steam, which turns a steam turbine, and the turbine cooperates with a generator to produce electrical power. The flue gas stream is subsequently passed through an air preheater, such as a rotating wheel heat exchanger that transfers heat from the flue gas to an incoming air stream, which thereafter flows to the combustor. The partially cooled flue gas is directed from the air preheater to the exhaust stack.
The flue gas contains contaminants such as nitrogen oxide (NOx) and carbon monoxide (CO) and particulates of soot when, for example, coal is used as the primary fuel source. The discharge of all of these contaminates into the atmosphere is subject to federal and local regulations, which greatly restrict the levels of these flue gas components.
To meet the reduced levels of NOx emissions from power stations, as required by environmental regulations, many fossil fuel-fired electric generating units are being equipped with selective catalytic reduction (SCR). In SCR, the most common method used is to inject ammonia or urea based reagents in the presence of a vanadium oxide catalyst where the ammonia reacts to reduce the oxides of nitrogen. The SCR process using ammonia occurs according to chemical equations (1) and (2):4NH3+4NO+O2→4N2+6H2O  (1)4NH3+2NO2+O2→3N2+6H2O  (2)
The SCR system typically operates at flue gas temperatures ranging between 300° C. and 450° C. U.S. Pat. No. 5,104,629 illustrates one known type of SCR installation.
One common problem with SCR technology is that some residual ammonia, known as ammonia slip, negatively impacts downstream components and processes such as: air pre-heater fouling, fly ash contamination, and ammonia gas emission into the atmosphere. An additional consequence of SCR technology is that increased ammonia injections will more efficiently remove the oxides of nitrogen, but then the excess ammonia will result in increased ammonia slip in the flue gas.
Because of regulated limits on the amount of ammonia that can be discharged to the atmosphere, there exists a need for a catalyst that can convert ammonia to nitrogen in an oxygen atmosphere without forming NOx. Moreover, there is a further need to reduce the number of catalysts needed at an installation. Therefore, a catalyst with dual functions of converting ammonia to nitrogen and carbon monoxide to carbon dioxide, without the use of a precious metal catalyst which converts carbon monoxide to carbon dioxide while promoting formation of NOx from ammonia, is desirable.
Further, there is an ongoing need for safe and efficient methods for minimizing ammonia slip downstream from an SCR catalyst.